Washing of paper pulp and the like



April 25, 1944. w. H. PORRITT WASH ING OF PAPER PULP AND THE LIKE Filed Sept. 11, 1941 great measure attained by maintaining the great- Patente'd Apr. 25, 1944 UNITED srArEs PATENT OFFICE William Henry Porritt, Chilcompton, near Bath,

England, assignor to Becker Partlngton a; company Limited, London, P

England, a British com- Application September 11, 1941, sci-m n6. 410,35:

In Great 8 Claims.

The present invention relates to the washin of paper pulp and similar fibrous materials. In

particular the invention relates to the washing of paper pulp such as may be produced by the disintegration of paper and in the course of deinking printed paper.

In the washing of paper pulp for whatever purpose, a number of desiderata exist which can be briefly summarized as follows:

(1) The washing must be effective, that is to say the finely divided or colloidal solid material together with soluble substances foreign to the pulp must be removed substantially uniformly and completely.

' (2) The requisite degree of cleanliness must 15 be attained with the minimum expenditure of water.

(3) The loss of fibre must be a minimum.

It has been found that these desiderata are in er part of the pulp in substantially uniform suspension and at the same time causing the washing liquid, e. g. water, to flow through the suspended pulp.

According to the present invention these conditions are achieved by providing with advantage a circular tank or container for the pulp which surrounds an upright fixed cylindrical screen or filter, adapted to prevent loss of fibre, mounted (ac-axially with the tank.

The screen or filter is surrounded near its external surface by a multiple bladed vertical agitator rotating coaxially with the screen or filter. The vertical blades are placed as close as is practically possible to the external surface of the screen. They are of relatively small cross sectional area so that slip occurs and are of a shape to produce good local eddy formation in the pulp. These two combined efiects give thorough scrubbing action between the fibres substantially only in the immediate vicinity of the screen or filter prior to the removal of the dirty water through the screen. Apart from this scrubbing action, the agitator blades also give rise 'to a slow rotation of the main body of the pulp.

It is also necessary to provide vertical spraypipes completely immersed in the pulp to give uniform introduction of the fresh water. These may be several in number and are placed vertically on the inside surface of the walls of the pulp container and are so constructed that the direction of the sprays is in the direction of the rotating pulp, so preventing clogging of the jets.

In operating the machine, the space inside 55 Britain SeptemberlZ, 1940 the screen or filter is filled with water to a level coinciding with that of the pulp outside. With these arrangements, the rotating multiple bladed agitator, in addition to scrubbing the fibres, centrifuges an annular cylinder of pulp in the immediate vicinity of the screen or filter, throwing the fibre outwards from the screen, thus facilitating the passage of dirty water through the screen and minimizing fibre loss. Both the scrubbing action, previously described, and the centrifugal action are substantially confined to the narrow annular column of pulp next to the screen, while the remaining pulp in the container is only in just suilicierit slow circulation to be maintained in uniform suspension.

Under these conditions, the fresh water introduced at the sides of the tank passes substantially horizontally through the pulp from the circumference towards the centre, carrying with it the fine particles of matter or other impurities, finally passing through the screen or filter when it can be drawn off through an adjustable valve at the bottom. I

The. dirty water must not be drawn oil at so great a rate as to cause a fall in the level in the centre compartment and to leave a rotating liquid pulp phase on the outside of the screen and a constantly increasing gaseous air phase on the inside of the screen which will be produced as the level is falling. This condition disturbs the centrifugal action of the agitator on the pulp, satisfactory production of which is dependent on a liquid phase on both sides of the screen over its whole operating area.

After passing through the screen or filter, any suspended matter tends to fall by gravity and. on account of the vertical positioning of the screen, has no chance of being reintroduced into the pulp.

In washing dirt and other foreign matter frompaper pulp, it is found that the rate of washing can be increased progressively as the pulp gradually becomes clean. In the method :of washing described, the rate of washing may be progressively maintained at its maximum rate as the operation proceeds by adjustment of the valves controlling the ingress and egress of water. These valves may be operated to this end either manually or by an automatic mechanical device.

Useful embodiments of apparatus according to the invention are shown by way-of example in the accompanying diagrammatic drawing .in which the same reference letters have been used to indicate the same or analogous parts.

I valve I.

Fig. 1 illustrates a washing apparatus of the upright type in a central sectional view;

Fig. 2 is a similiar representation of an upright washing apparatus of a modified construction.

In the construction shown in Fig. 1 the vessel or tank A in which the washing is performed is substantially cylindrical and is provided concentrically therewith with a. cylindrical screen B of smaller diameter, the charge of pulp P being introduced into the annular space S between the two cylinders. Water is admitted to this annular space through a valve I from a supply pipe I by which the rate of fiow is regulated. Preferably the water is admitted and distributed uniformly through the .vertical column of the pulp in the form of a plurality of substantially horizontal jets by way of a plurality of openings provided at different levels preferably in a spray pipe 0 extending substantially to the bottom of the pulp container A. To enhance uniform distribution in the horizontal plane a number of such spray pipes C may be provided at equal spacings around the periphery of the vessel A. The screen B is of a suitable mesh to prevent the passage of the fibre while permitting the passage of the water and of the impurities it is desired to remove by washing. We have used with good effect a screen constituted by a wire screen of 66 mesh. As this itself may not have the necessary mechanical strength it may with advantage be supported in. any convenient way. The water passing through the screen escapes at the bottom of the vessel. Two alternative outlets D and E are provided one directly by way of a valve 2 and the other by way of a standpipe E. The latter outlet E is also connected to the water inlet I by way of a valve 3 forming a priming valve. The valve .2 is so adjusted that the amount of water passing to waste is slightly less than the amount of water introduced through the Consequently a certain quantity of water flows through the standpipe E and over an associated weir device W thereby maintaining a substantially constant level within the cylindrical screen B. For a screen 3' in diameter, I2 agitator arms F will be found to be satisfactory, i. e., the peripheral spacing is of the order of 9". The arms F are each preferably in the form of a vertical rod which may be of circular cross section and are secured to a rotor G so as to extend close to the screen B and in this instance shown to be driven by a bevel gear G. A suitable spacing between the rods and screen is of the order of 1". During the washing operation the agitator arms F are rotated at an even speed for example 60 revolutions per minute when the screen is 3' in diameter. For screens of different diameter the number of revolutions should vary inversely with the square root of the diameter. The jets of water from the spray pipe or pipes C are directed in the same sense as the rotation of the rotor G. For a vessel A capable of carrying a charge of 400 lbs. of fibre (dry basis) and fitted with a screen 3' in diameter and 6' high washing can be effected in about 45 minutes.

If during the washing operation the hydrostatic balance on opposite sides of the screen B should become upset, the screen may become clogged which will become apparent from an immediate fall of the level of the water inside the screen. When clogging occurs the valves I and 2 are closed and the valve 3 is opened to introduce fresh water to the' outlet side of the screen.

When the water and pulp levels are 'ebalanced the screen will become clear and the normal operation can be resumed.

When the washing is complete, the supply of water is cut of! by the closure of the valve I and after the waste water has been drawn oil! through the valve 2 the pulp is drawn off through the pulp outlet 4. A fresh charge can then be introduced and a further washing operation performed.

The nature of the agitators F should be such as to prevent any breaking up of the fibre under treatment for which purpose sharp edges thereon should be avoided and in addition if desired the agitator arms F may be rubber covered. The screen can if desired be made exchangeable in order to allow, by selection of screens of the requisite mesh, optimum treatment of pulp of different fibre length.

In the constructional form of apparatus illustrated in Fig. 2 the screen B is of a rotary type. In this case provision is made between the outer surface of the screen B and the agitator members F, of a number of vertical fixed rods R which are positioned close to the surface of the screen B and by preference are bolted to the bottom and sides of the pulp container A.- In

this instance the screen B is adapted to be rotated with the agitator arms F and to that end is connected to the agitator arms F substantially midway by a central spider F. The rotor G in this case is again shown to be driven by a bevel gear G.

The pulp inside the vessel A is thus again agitated centrifugally more particularly in proximity of the screen. The fixed vertical rods R here used tend to retard this motion and accordingly the velocity of the pulp movement in the immediate vicinity of and in relation to that of the screen whereby an eflicient agitation and continuous mixing up of the pulp column with the water is always maintained. Fresh wash water in this instance may be admitted by means similar to those shown in Fig. 1 or directly from above the pulp vessel A while the waste water may be drawn off through the socket connection shown at D again preferably over a weir W with the result as explained by reference to Fig. 1 that similar conditions will be set up near the surface of the rotatable screen B as in the case of a stationary washer screen associated with rotating agitator arms.

It will be understood that although reference has been made to washing water, the use of other liquids or of solutions for the treatment of pulp and the like by the application of the principles set out above is not excluded.

Although in the foregoing the invention has been described as applying to batch washing, it may obviously be operated either on a circulatory or straight-through flow system. I

I claim:

1. Pulp washing apparatus comprising in combination a stationary outer vessel, an inner screening vessel coaxially therewith, an innular space for the pulp charge between said vessels, rotary agitating means arranged in proximity to and circumferentially about said screening vessel, means for drawing on washing liquid from the space within said inner screening vessel and means for introducing washing liquid into said pulp charge from a plurality of juxtaposed points transversely to the pulp column subtsantially as set forth.

2. Pulp washing apparatus comprising in combination a stationary outer vessel, an inner screening vessel coaxial therewith, an annular space for the pulp charge between said vessels, agitating means arranged in proximity to and circumferentially about said screening vessel, means for rotating said agitating means, means for drawing of! washing liquid from the space within said inner screening vessel and a plurality of conduits extending through the pulp column close to the inner wall of the outer vessel for introducing washing liquid in the form of several juxtaposed jets transversely through the pulp column substantially as set forth.

3. Pulp washing apparatus comprising in combination a stationary outer vessel, an inner screening vessel coaxial therewith, an annular space for the pulp -charge between said vessels, rotary.

agitating means constituted by a series of beater arms arranged circumferentially about said screening vessel over the whole length thereof, means for drawing of! washing liquid from the space within said inner screening vessel and .a plurality of pipes extending through the pulp column close to the wall of the outer vessel and provided with a series of apertures shaped to introduce the washing liquid transversely through the pulp column in the direction of rotation of the pulp therein substantially as set forth.

4. Pulp washing apparatus comprising in combination a stationary outer vessel, an inner rotatable screening vessel arranged coaxially therewith, an annular space for the pulp charge between said vessels, rotaryagitating means arranged in proximity to and circumferentially about said screening vessels, means for drawing oi! washing liquid from the space within-said inner screening vessel and means in said annular space for introducing washing liquid into the bination a stationary outer vessel, a rotatable inner screening vessel coaxial therewith, an anpulp charge transversely to the pulp column formed therein substantially as set forth.

5. Pulp washing apparatus comprising in combination a stationary outer vessel a rotatable inner screening vessel arranged coaxially therewith, an annular space between said vessels for the pulp charge, agitating means in the form of a plurality of beater arms extending parallel with and circumferentially about said rotary screening vessel and adapted to rotate therewith in unison, means for drawing off washing liquid from the space within said inner screening vessel, means in said annular space for introducing washing liquid in several juxtaposed layers transversely through the pulp column therein 1 and a plurality of stationary baflles arranged circumiermtially about said screening vessel between the latter and said rotary agitator arms substantially as and for the purpose set forth.

6. Pulp washing apparatus comprising in comnular space between said vessels for the pulp charge, a plurality of agitating arms extending parallel with and circumierentially about said screening vessel and connected to the latter, a plurality of stationary rod shaped baflles arranged circumferentially about. and parallel with the screening vessel between the latter and the rotary agitating arms, means for rotating said screening vessel in unison with said agitator arms, means for drawing oft washing liquid from the space within said inner screening vessel and a plurality of spray pipes in said annular space close to the walls of said outer vessel for introdusing washing liquid transversely through the pulp column therein from a plurality of juxtaposed openings in the direction of rotation of the pulp substantially as and for the purpose set forth.

7. Pulp washing apparatus comprising in combination an outer vessel, an inner screening ves-- sel arranged coaxially therewith, an annular space for the pulp between said vessels, agitating means arranged in proximity to and circumferentially about said creening vessel, means in said annular space for introducing washing liquid into the pulp transversely through the pulp column therein,means for drawing oil washing liquid from the space within said inner screening vessel and means for maintaining the level in said outer and inner vessels practically constant substantially as and for the purpose set forth.

8. Pulp washing apparatus comprising in combination an outer vessel, an inner screening vessel coaxial therewith, an annular space for the pulp charge between said vessels, rotary agitating means arranged circumferentially about said screening vessel and extending through the pulp column, an opening in the bottom of the outer vessel for the discharge of the finished pulp from said annular space, a central opening for the discharge of spent washing liquid from the bottom of said screening vessel, a plurality of conduits extending through the pulp column in said annular space for introducing washing liquid transversely through said pulp column, a supply conduit for the supply of washing liquid thereto, a secondary outlet from the bottom of said screening vessel, a standpipe between said supply conduit and the pipe connected to said secondary outlet, a weir in communication with the pipe of said secondary outlet and the conduit connected 

